This invention relates to current limiting fuses having parallel electrical weak spots formed in the fusible strip thereof, and more particularly to such fuses wherein the parallel electrical weak spots are formed transverse to the length of said fusible strips.
Fuses including thin metal fusible strips having multiple punched sections defining electrical weak spots are well known in the art and are presently used extensively. Such fusible strips most often are found in enclosed cartridge type electric fuses. Electrical weak spots are formed in constricted areas along the electrical path of the fusible strip, generally between two adjacent apertures defined therein or between an aperture and an adjacent edge of the fusible strip such electrical weak spots are areas which are less conductive than the remaining areas of the strip. The apertures most commonly are long and narrow with relatively sharp corners and are positioned to lie transverse to the electrical path. Such apertures, and thereby the electrical weak spots, in the fusible strip are formed by removing material from strip stock through the use of special punch dies. Problems have been encountered in the manufacture of great quantities of such fusible strips because the punch dies required to form electrical weak spots having such configurations are often intricate, resulting in weakened structures which require considerable maintenance and frequent replacement.
Furthermore, the punch dies produce stress areas in the thin metal strip material during the punching process. The stress areas tend to physically weaken the fusible strip particularly in the area of the electrical weak spots which experience constant expansion and contraction due to heating and cooling of the strip during normal electrical operation. Such physical weakening of the fusible strip can cause premature failure thereof.
In addition, prior art fusible strips commonly contain unnecessarily large amounts of strip material in the vicinity of the electrical weak spots. While this does not necessarily effect fuse clearing characteristics, it may allow additional damaging current to pass during a short circuit or heavy overload, thereby increasing the fuse I.sup.2 t. Excess material in the vicinity of the arc which forms at the electrical weak spots deters rapid arc elongation causing lower arc voltages and higher currents.